Support field presence for in64 types

Author: roblabla

src/betterproto/__init__.py

@@ -1075,6 +1075,8 @@ def to_dict(
                 if meta.proto_type in INT_64_TYPES:
                     if field_is_repeated:
                         output[cased_name] = [str(n) for n in value]
+                    elif value is None:
+                        output[cased_name] = value
                     else:
                         output[cased_name] = str(value)
                 elif meta.proto_type == TYPE_BYTES: